The present invention relates to an apparatus for welding components together with the use of ultrasound, and particularly to an apparatus which employs a sonotrode to focus mechanical ultrasonic vibrations on the components to be welded.
Ultrasonic welding of contact materials, for example, the welding of silver and silver alloys onto substrates of copper and copper alloys, as well as aluminum, by means of ultrasound has been known in the art for some time (see, for example, "Zeitschrift ur Werkstofftechnik" [Journal of Material Technology], Volume 6, No. 4, 1975, pages 125-137). Ultrasonic welding employs a combination of friction and cold pressure welding applied at overlapped surfaces of parts to be welded together. A certain contact pressure is assumed to exist during ultrasonic welding. With the simultaneous application of contact pressure forces and ultrasonic energy, the required forces, and consequently also the developing deformation at the parts being welded, are reduced in an advantageous manner.
Due to high frequency frictional vibrations, which are the local effect of ultrasonic energy, the oxide layers and other impurities existing at the surfaces of the materials are destroyed without incurring noticeable plastic deformation in the region of the weld. The thus purified metal surfaces weld together at relatively low pressure due to the presence of metallic bonding forces.
One principle of the ultrasonic welding machine is described on page 127 of the above-identified reference. Ultrasonic welding devices basically include a high frequency generator (frequency converter), a sonic head (vibration converter), a sonotrode (vibration conductor), a machine frame to accommodate the parts to be welded and operating elements. The oscillations of high frequency current generated by the high frequency generator are converted into mechanical vibrations by a so-called sonic head which acts as an electroacoustical transducer. The electrical high frequency oscillations furnished by the high frequency generator are thus converted to mechanical vibrations at the same frequency. The sonotrode amplifies the vibrations and transmits them to the material to be welded.
An ultrasonic metal welding machine made by TELSONIC/ Switzerland (Model MSM-700, brochure dated July, 1977) is used to connect different types of materials if their hardness differences are not too great. For example, it can be used to connect shaped aluminum foils of electrolyte capacitor windings to a housing or to contact members, or to connect coil ends of electrical instruments, or to weld aluminum foils. All important welding parameters, such as vibration amplitude, pressure, welding time, delay time, holding time and carriage advance can be varied with accuracy.
It is also known to fasten solar cell connectors by means of ultrasonic welds to solar cell contacts (U.S. Pat. No. 3,620,847). The above-mentioned ultrasonic welding machines, in which a sonotrode is applied at a defined force by means of system-defined mechanisms, can be used for this purpose wherein an ultrasonic pulse is actuated and the further parameters such as welding time, contact pressure, contact surface and pulse amplitude are varied to produce the desired weld. To form a module of cells, with each single cell being either 5.times.5 or 10.times.10 cm.sup.2, up to 16 individual steps per cell, i.e. individual welds, need to be made to interconnect the cells into a module. A particular drawback in this operation is that, in order to realize a mechanically perfect weld between contact and cell (an approximately 80 .mu. thick aluminum foil), the sonotrode must have a defined vibration amplitude. This amplitude, in conjunction with the required contact pressure force, causes the solar cells to vibrate as well at the vibration frequency of, e.g., 36 kHz, which then often leads to breaks in the solar cell. When welding the cells into modules, this involves, in addition to many rejects, considerably more manual work in connection with the module welding which is normally performed automatically.